1. Field of the Invention
The present invention relates to the structure of an air bag door of an instrument panel, and particularly, to the structure of an air bag door that is integral with an instrument panel.
2. Description of the Related Art
FIG. 16 shows an instrument panel P of a car. The instrument panel is arranged inside the car under a windshield, to hold instruments and protect persons in the front seat. The instrument panel is generally made of a base, a skin, and a polyurethane foam body formed between the base and the skin. A simple way to form the instrument panel is to arrange a skin and a base in molds, inject polyurethane foam ingredient between the skin and the base, and make the foam ingredient foam and solidify with the skin and base. This technique is called a monolithic molding technique.
Recent instrument panels are usually provided with an air bag device for the seat next to the driver's seat. FIG. 17 is a sectional view taken along a line a--a of FIG. 16 and shows an air bag device fitted to the instrument panel of FIG. 16. A folded air bag A and an igniter I are housed in a casing C, to form the air bag device. The air bag device is fitted to the back of the instrument panel P so that the air bag device faces the seat next to the driver's seat. The base 74 has an opening O above the casing C, to let the air bag A inflate. The opening O is covered with an air bag door D. Upon occurrence of a collision the car, the igniter I inflates the air bag A, which pushes and opens the door D from the back thereof and expands into the inside of the car.
The air bag door D may be separately prepared and fitted to the instrument panel later, or it may be integrally formed with the instrument panel.
Separately preparing the door D involves troublesome fitting work and fitting errors. On the other hand, integrally forming the door D has no such problems. The present invention relates to the latter method.
The instrument panel of the prior art of FIGS. 16 and 17 has the integral air bag door D. FIG. 18 is an enlarged view showing an encircled part of FIG. 17. The instrument panel P is formed according to the monolithic molding technique and is composed of the base 74, an air bag door core 75, the skin 72, and the foam body 71. The base 74 has the opening O for passing the air bag. The door core 75 is placed on the base 74, to close the opening O. The skin 72 has a breaking facility 73 that defines the air bag door D. The foam body 71 fills a space between the base 74 and the skin 72.
The breaking facility 73 is a notch or a V-shaped groove formed by thinning the skin 72 by high-frequency welder, hot blade, cold press cutter, etc. A reference mark T is an expected break line that is an imaginary line passing through the foam body 71.
If inflated, the air bag A pushes the air bag door D. Stress due to the inflating air bag appears at the breaking facility 73 and breaks the foam body 71 and breaking facility 73, to open the air bag door D and expand the air bag A into the inside of the car as shown in FIG. 19. In FIG. 19, the skin 72 has been broken at parts 73a and 73b, and the foam body 71 at parts Ta and Tb.
The expected break line T is a shortest ideal line achieved when the foam body 71 breaks most efficiently in a shortest time. The line T is only an imaginary line and is not a real line formed of, for example, a slit. The actual broken parts Ta and Tb are usually irregular and differ from the ideal line T. Namely, the air bag door D is irregularly opened and the foam body 71 is irregularly broken to scatter.
It is difficult to form a notch or a slit along the line T in the foam body 71 because the foam body 71 is made from polyurethane foam ingredient between the base 74 and the skin 72.
The polyurethane foam ingredient frequently leaks from between the opening O and the door core 75, to cause an underfill in the foam body 71. To prevent this, more polyurethane foam ingredient is needed. If the ingredient leaks, extra work of removing burrs formed by the leaked ingredient will be necessary.